Generally, when a wireless signal is transmitted over-the-air, reflections of the signal will occur whenever the signal encounters a solid surface, for example, buildings, bridges, large rocks (hills and mountains), walls, etc. The original wireless signal and the reflections will then scatter and arrive at a destination at different times. This is commonly referred to as multipath. Each time that a reflection occurs, a net reduction of the original wireless signal's strength is seen, since the surface that the original wireless signal reflects from absorbs a portion of the original wireless signal and the reflection itself takes away a part of the original wireless signal.
Multipath can be a problem when it comes to the proper reception of the transmitted signal, with the reflections interfering with the original wireless signal and degrading system performance. Multipath occurs in many wireless communications systems, from uni-directional systems such as AM/FM radio to bi-directional systems like cellular telephones and geosynchronous satellite based communications systems. In an ultra-wideband (UWB) communications system that may use a stream of short-duration pulses, the reflections of the pulses can be readily distinguished from the original pulse due to the fact that the pulses are of short-duration and there is normally no mingling of the original pulse and any of the reflections. In wireless communications systems with a continuously transmitted signal or wherein the signal length is longer than a typical multipath delay, problems may arise from the intermingling of the original signal and the reflections and special techniques must be employed to separate the signal from the reflections.
However, in systems where the multipath delay is greater than the signal length, it is possible to easily combine the received original wireless signal (attenuated by distance traveled and reflections lost) with the received reflections to maximize the received signal strength. A commonly used technique employs what is commonly referred to as a rake receiver. The rake receiver has multiple “fingers” that measures the received signal at various points in time, hoping to capture the reflections of the original wireless signal. The signals measured by the fingers are then combined so that the received signal strength is maximized.
Rake receivers are commonly implemented in software and make use of existing receivers. This permits the benefits of rake receivers without significantly increasing the cost of the receiver or requiring the re-design of a receiver without a rake receiver to include the rake receiver.
Additionally, in wireless communications systems that use the precise arrival of signals and pulses, a slight mismatch between the actual arrival time of the signal and the expected arrival time of the signal can result in a significant reduction in the received signal strength. This is often due to a misalignment between the signal and a correlator/integrator that is used to detect the arrival of the signal and to produce a value that is proportional to the received signal strength of the signal. It is therefore, vital to align the correlator/integrator with the received signal to maximize the available received signal strength.
However, since the arrival times of the signal can vary, either on purpose (such as due to a modulation scheme) or through error (via clock drift, etc.), a method of simply using historical timing information derived from previously arrived signals is generally not adequate. Even techniques that use the most recently received signal to predict the arrival of the next signal may not provide optimal performance. This is especially true in an ultra-wideband (UWB) wireless communications system that makes use of a stream of short-duration pulses to encode and convey information. Depending on the modulation scheme used, the actual arrival time of the short-duration pulses may be varied to convey information.
One disadvantage of the prior art is that many of the rake receivers are implemented via software, using existing receiver hardware. While this can minimize any additional hardware costs involved with the implementation of a rake receiver, the fact that the rake receiver is implemented in software can place limits on a maximum speed at which the rake receiver can operate. Therefore, if the incoming signals are arriving too quickly, then the software rake receiver may not be able to measure a significant number of the reflections to significantly improve the received signal strength.
A second disadvantage of the prior art is that while existing rake receivers can increase the received signal strength by combining reflections of the received signal, if the received signals and the reflections themselves are not at a maximum signal strength, then system performance can suffer.
Yet another disadvantage of the prior art is that historical timing information cannot be reliably used to detect the arrival of future signals. This is due to problems associated with clock drive and even modulation schemes.